[9.x] Add key support to FormRequest validated method

[benholmen]

When interacting with FormRequests and using validated data it's a little clunky to get single values. This PR extends the current FormRequest@validated method to make this more expressive.

You can currently get a single validated value with the following:

// this does not seem obviously validated:
$name = $request->input('name');

// ouch
$name = $request->validated()['name'];

I think it's more expressive and clean to use:

$name = $request->validated('name');